Color projection apparatus are known that employ multiple light-valve panels such as liquid-crystal light valves (LCLVs).
One example of such an apparatus is shown in FIG. 1, in which white light (comprising the three primary colors of red (R), green (G), and blue (B)) from a light source (lamp 1) and converted into a substantially parallel white light flux by a concave mirror 2 and a condenser lens 3. The white light flux enters a color-separating optical system comprising a blue-light (B light) reflecting dichroic mirror 4 and a green-light (G light) reflecting dichroic mirror 5. B light reflected by the B-light-reflecting dichroic mirror 4 is reflected again by a mirror 7 and enters a B-light LCLV 11. G light reflected by the G-light-reflecting dichroic mirror 5 enters a G-light LCLV 10. Red light (R light) passing through the dichroic mirror 5 is reflected by a mirror 6 and a mirror 8 and enters an R-light LCLV 9. Each color of light entering the respective LCLV is modulated by the respective LCLV. Hence, each color's video signal is converted into an image that has a transmission-rate distribution at the respective LCLV. The modulated colored lights passing through the respective LCLVs enter a dichroic prism 12. The dichroic prism 12 comprises a reflecting dichroic film for R light and a reflective dichroic film for B light, thereby accomplishing a three-color combination. The color-combined light exits from the dichroic prism 12 and is magnified and projected on a screen (not shown) by a projection lens 13.
In the configuration shown in FIG. 1, about half the light energy entering the LCLVs 9-11 is absorbed and converted into heat. Thus, a problem with the conventional example shown in FIG. 1 is its inability to produce a sufficiently bright projected image for certain applications.
Another shortcoming of the foregoing configuration and other configurations in the prior art is that the angle of incidence of rays on the dichroic mirrors is not identical over the entire surface of each such mirror. Optical components comprising multilayered films, such as dichroic mirrors and dichroic prisms, are highly angle-dependent in their spectral characteristics. Consequently, whenever the angle of incidence of a principal ray (the principal ray being defined by the projection lens) in relation to a multilayer film is not exactly the same at all locations on the multilayer film, the multilayer film's spectral characteristics are not the same for each such principal ray, resulting in problems with color shading on the projection screen.
LCLVs and related devices are also angle-dependent. Whenever the angle of incidence of a principal ray in relation to a LCLV differs even slightly across the surface of the LCLV, as is the case with prior-art projection apparatus, problems with uneven contrast arise in the projected image.
An alternative configuration to that shown in FIG. 1 is also known in the prior art, in which dichroic mirrors corresponding to the mirrors 4 and 5 are positioned in an X pattern relative to each other rather than parallel to each other. The X pattern is characteristic of so-called "crossed dichroic" mirrors. Unfortunately, with a crossed-dichroic mirror, the intersecting portions of the two dichroic mirrors exhibit color shift relative to other portions of the mirrors. This uneven-color problem affects image quality.
A conventional color-projection apparatus intended for producing an image with enhanced brightness is disclosed in the first figure of Japanese Kokai patent document no. Hei 4-18544, in which a polarizing beamsplitter (PBS) is employed to "split" a white light flux into an S-polarized flux component and a P-polarized flux component. The P-polarized flux component is transmitted through the PBS, and the S-polarized component is reflected by the PBS. The P-polarized component and the S-polarized component are each color-separated into polarized R, G, and B lights by a dichroic mirror. Each of the polarized color fluxes is modulated by a respective transmissive-type LCLV. If a polarizing plate at each LCLV's exit side is situated orthogonally to the incident side, the P-polarized light is converted into S-polarized light and the S-polarized light is converted into P-polarized light. The modulated polarized color fluxes are recombined using dichroic mirrors and a second PBS; the resulting image is projected using a projection lens.
Whereas the image projected by an apparatus according to Kokai '544 is definitely brighter than projected images produced using apparatus that convert substantial amounts of light energy to heat, as described above, this apparatus requires six LCLVs, one for each primary color of S- and P-polarized light. Also, each polarized component of each color of light requires the same number of lenses. Consequently, this apparatus is very costly.
Yet another prior-art color projection apparatus (as disclosed in the first figure of Japanese Kokai patent document no. Hei 3-296030) is shown in FIG. 2. Light flux from a light source 21 is reflected by a curved mirror 22 (to make the rays parallel to each other) and split by a first polarizing beamsplitter A(PBS) 23 into a P-polarized light flux and an S-polarized light flux. The P-polarized flux is separated into the three primary colors SR, SG, and SB by the dichroic mirrors 24 and 25. LCLVs 26, 27, 28 modulate the respective R, G, and B lights SR, SG, SB (according to respective color-difference signals R-Y, G-Y, and B-Y provided to respective terminals 29, 30, and 31). Routing of the R, G, and B lights is performed by mirrors 32, 33. Meanwhile, the other polarized light flux produced by the first PBS 23 is reflected by a mirror 34, enters a "luminance-signal" LCLV 35, and is modulated according to a luminance signal Y provided to a terminal 36. The three colored modulated lights LR, LG, LB (recombined by dichroic mirrors 37, 38) and the modulated luminance light LY are integrated by a second PBS 39. The light is then projected through a projection lens 40 onto a screen (not shown). Thus, the number of LCLVs required in this configuration is four rather than six.
In the Japanese Kokai patent document no. Hei 3-296030 summarized above, the image produced by the luminance-signal LCLV is superimposed on the superimposed images produced by the color-signal LCLVs to enhance luminance (brightness) of the projected image. Preferably, the LCLVs used for each of the primary colors and the LCLV used for the luminance signal have the same size and shape.
Further with respect to the Kokai '030 apparatus, resolution of the projected image could be improved by using LCLVs having higher resolution; i.e., by using LCLVs having a greater number of pixels per unit LCLV area. This could be achieved by reducing pixel size. But, there are manufacturing limitations on the minimum pixel size in LCLVs. A higher-resolution LCLV must be larger in proportion to its resolution. When the LCLVs used for the primary colors are individually the same size as the luminance-signal LCLV, all the LCLVs used in the embodiment described above would have to be enlarged in order to improve resolution. Consequently, cost would be substantially increased. Also, the overall size of the projection apparatus (including an increase in size of optical components) would have to be correspondingly increased with an increase in size of the LCLVs. This would unavoidably lead to substantially higher costs despite the use of only four LCLVs instead of six.
Other problems arise when the shape and size of the image-forming portion of each of the color-signal LCLVs is identical to the shape and size of the image-forming portion of the luminance-signal LCLV. If the numerical aperture (i.e., ratio of the surface area of the portion through which incident light can be transmitted to the surface area of the portion through which incident light cannot be transmitted, such as a portion where a switching element is located) of each color-signal LCLV and the luminance-signal LCLV are identical, it is impossible to obtain both a high-resolution and a bright projected image. Because there is a limit on the size of a switching element provided for each pixel in a LCLV, the number of pixels generally cannot be increased while also maintaining a large numerical aperture. Therefore, if the shape and size of the image-forming portion of a color-signal LCLV is identical to the shape and size of the image-forming portion of the luminance-signal LCLV, and if the number of pixels of each type of LCLV is increased, the numerical aperture of both types drops. The projected image will have a higher resolution but will be darker. In contrast, if the number of pixels of each type of LCLV is decreased while the numerical aperture of both types of LCLVs is increased, the projected image will be brighter but the resolution will be lower.
Further with respect to the Kokai '030 apparatus, greater brightness could be achieved by increasing the resolution of the luminance-signal LCLV 35 relative to the color-signal LCLVs 26, 27, 28.
The Kokai '030 apparatus shown in FIG. 2 has certain problems. First, each dichroic mirror and dichroic prism used in the apparatus comprises a multiple-layer film. The spectral characteristics of the films used in the dichroic mirrors and prisms, as used for color separation or color combining, exhibit an angular dependence. I.e., the angle of incidence differs depending on the location, with respect to the principal ray, of the multiple-layer film as determined by the aperture stop of the projection lens. As a result, the spectral characteristics of the multiple-layer film will be different for each principal ray. This results in color shading on the screen.
Second, the performance of each LCLV is dependent on the angle of incidence of light with respect to the LCLV. Since the angle of incidence of a principal ray is different depending on the location of the principal ray with respect to a LCLV, an uneven contrast of the projected image is exhibited by the apparatus of Kokai '030.
Third, there is an angular dependence on the performance of the polarizing beamsplitter used for separating or combining polarized light in the Kokai '030 apparatus. Any difference in the angle of incidence of the principal ray depending on its location with respect to the polarizing-and-splitting surface of a polarizing beamsplitter results in uneven contrast of the projected image.
Another problem with the configuration shown in FIG. 2 pertains to the PBSs 23, 39. In these PBSs, a multi-layered dielectric film is provided on a supporting optical member such as glass. Increasing the size of the optical components in order to accommodate an increase in size of the LCLVs necessitates a corresponding increase in the size of the dielectric films. This results in an appreciable increase in the cost of the supporting optical members, which must have homogeneous optical qualities. This results in high cost and procurement difficulties.